The present invention relates to the art of electric arc welding and more particularly to an electric arc welding system for combining power supplies to operate tandem electrodes.
The present invention is directed to an electric arc welding system utilizing high capacity alternating circuit power supplies for driving two or more tandem electrodes of the type used in seam welding of large metal blanks. Although the invention can be used with any standard AC power supply with switches for changing the output polarity, it is preferred that the power supplies use the switching concept disclosed in Stava U.S. Pat. No. 6,111,216 wherein the power supply is an inverter having two large output polarity switches with the arc current being reduced before the switches reverse the polarity. Consequently, the term xe2x80x9cswitching pointxe2x80x9d is a complex procedure whereby the power supply is first turned off awaiting a current less than a preselected value, such as 100 amperes. Upon reaching the 100 ampere threshold, the output switches of the power supply are reversed to reverse the polarity from the D.C. output link of the inverter. Thus, the xe2x80x9cswitching pointxe2x80x9d is an off output command, known as a xe2x80x9ckillxe2x80x9d command, to the power supply inverter followed by a switching command to reverse the output polarity. The kill output can be a drop to a decreased current level. This procedure is duplicated at each successive polarity reversal so the AC power supply reverses polarity only at a low current. In this manner, snubbing circuits for the output polarity controlling switches are reduced in size or eliminated. Since this switching concept is preferred to define the switching points as used in the present invention, Stava U.S. Pat. No. 6,111,216 is incorporated by reference. The concept of an AC current for tandem electrodes is well known in the art. Prior application Ser. No. 09/336,804 filed Jun. 12, 1999 discloses a system whereby tandem electrodes are each powered by a separate inverter type power supply. The frequency is varied to reduce the interference between alternating current in the adjacent tandem electrodes. Indeed, this application relates to single power sources for driving either a DC powered electrode followed by an AC electrode or two or more AC driven electrodes. In each instance, a separate inverter type power supply is used for each electrode and, in the alternating current high capacity power supplies, the switching point concept of Stava U.S. Pat. No. 6,111,216 is employed. This system for separately driving each of the tandem electrodes by a separate high capacity power supply is background information to the present invention and is incorporated herein as such background. In a like manner, U.S. application Ser. No. 09/406,406 filed Sep. 27, 1999 discloses a further arc welding system wherein each electrode in a tandem welding operation is driven by two or more independent power supplies connected in parallel with a single electrode arc. The system involves a single set of switches having two or more accurately balanced power supplies forming the input to the polarity reversing switch network operated in accordance with Stava U.S. Pat. No. 6,111,216. Each of the power supplies is driven by a single command signal and, therefore, shares the identical current value combined and directed through the polarity reversing switches. This type system requires large polarity reversing switches since all of the current to the electrode is passed through a single set of switches. This application does show a master and slave combination of power supplies for a single electrode and discloses general background information to which the invention is directed. For that reason, this application is also incorporated by reference.
Welding applications, such as pipe welding, often require high currents and use several arcs created by tandem electrodes. Such welding systems are quite prone to certain inconsistencies caused by arc disturbances due to magnetic interaction between two adjacent tandem electrodes. A system for correcting the disadvantages caused by adjacent AC driven tandem electrodes is disclosed in prior application Ser. No. 09/336,804 filed Jun. 21, 1999 by assignee of this invention. In that prior application, each of the AC driven electrodes has its own inverter based power supply. The output frequency of each power supply is varied so as to prevent magnetic interference between adjacent electrodes. This system requires a separate power supply for each electrode. As the current demand for a given electrode exceeds the current rating of the inverter based power supply, a new power supply must be designed, engineered and manufactured. Thus, such system for operating tandem welding electrodes require high capacity or high rated power supplies to obtain high current as required for pipe welding. To decrease the need for special high current rated power supplies for tandem operated electrodes, assignee developed the system disclosed in application Ser. No. 09/406,406 wherein each AC electrode is driven by two or more inverter power supplies connected in parallel. These parallel power supplies have their output current combined at the input side of a polarity switching network. Thus, as higher currents are required for a given electrode, two or more parallel power supplies are used. In this system, each of the power supplies are operated in unison and share equally the output current. Thus, the current required by changes in the welding conditions can be provided only by the over current rating of a single unit. A current balanced system did allow for the combination of several smaller power supplies; however, the power supplies had to be connected in parallel on the input side of the polarity reversing switching network. As such, large switches were required for each electrode. Consequently, such system overcame the disadvantage of requiring special power supplies for each electrode in a tandem welding operation of the type used in pipe welding; but, there is still the disadvantage that the switches must be quite large and the input, paralleled power supplies must be accurately matched by being driven from a single current command signal. This prior application does utilize the concept of a synchronizing signal for each welding cell directing current to each tandem electrode. However, the system still required large switches. This type of system was available for operation in an ethernet network interconnecting the welding cells. In ethernet interconnections, the timing cannot be accurately controlled. In the system described, the switch timing for a given electrode need only be shifted on a time basis, but need not be accurately identified for a specific time. Thus, the described system requiring balancing the current and a single switch network has been the manner of obtaining high capacity current for use in tandem arc welding operations when using an ethernet network or an internet and ethernet control system. There is a desire to control welders by an ethernet network, with or without an internet link. Due to timing limitation, these networks dictated use of tandem electrode systems of the type using only general synchronizing techniques.
It is advantageous in high current systems for arc welding to drive one electrode with several paralleled inverter type power supplies while accommodating network control. The disadvantage has been that such systems required the current to be accurately balanced and required a single high capacity output switching network. Such systems could be controlled by a network; however, the parameter to each paralleled power supply could not be varied. Each of the cells could only be offset from each other by a synchronizing signal. Such systems were not suitable for central control by the internet and/or local area network control because an elaborate network to merely provide offset between cells was not advantageous.
The present invention utilizes the concept of a single AC arc welding cell for each electrode wherein the cell itself includes one or more paralleled power supplies each of which has its own switching network. The output of the switching network is then combined to drive the electrode. This allows the use of relatively small switches for polarity reversing of the individual power supplies paralleled in the system. In addition, relatively small power supplies can be paralleled to build a high current input to each of several electrodes used in a tandem welding operation. The use of several independently controlled power supplies paralleled after the polarity switch network for driving a single electrode allows advantageous use of a network, such as the internet or ethernet.
In accordance with the invention, smaller power supplies in each system are connected in parallel to power a single electrode. By coordinating switching points of each paralleled power supply with a high accuracy interface, the AC output current is the sum of currents from the paralleled power supplies without combination before the polarity switches. By using this concept, the ethernet network, with or without an internet link, can control the weld parameters of each paralleled power supply of the welding system. The timing of the switch points is accurately controlled by the novel interface, whereas the weld parameters directed to the controller for each power supply can be provided by an ethernet network which has no accurate time basis. Thus, an internet link can be used to direct parameters to the individual power supply controllers of the welding system for driving a single electrode. There is no need for a time based accuracy of these weld parameters coded for each power supply. In the preferred implementation, the switch point is a xe2x80x9ckillxe2x80x9d command awaiting detection of a current drop below a minimum threshold, such as 100 amperes. When each power supply has a switch command, then they switch. The switch points between parallel power supplies, whether instantaneous or a sequence involving a xe2x80x9ckillxe2x80x9d command with a wait delay, are coordinated accurately by an interface card having an accuracy of less than 10 xcexcs and preferably in the range of 1-5 xcexcs. This timing accuracy coordinates and matches the switching operation in the paralleled power supplies to coordinate the AC output current.
By using the internet or ethernet local area network, the set of weld parameters for each power supply is available on a less accurate information network, to which the controllers for the paralleled power supplies are interconnected with a high accuracy digital interface card. Thus, the switching of the individual, paralleled power supplies of the system is coordinated. This is an advantage allowing use of the internet and local area network control of a welding system. The information network includes synchronizing signals for initiating several arc welding systems connected to several electrodes in a tandem welding operation in a selected phase relationship. Each of the welding systems of an electrode has individual switch points accurately controlled while the systems are shifted or delayed to prevent magnetic interference between different electrodes. This allows driving of several AC electrodes using a common information network. The invention is especially useful for paralleled power supplies to power a given electrode with AC current. The switch points are coordinated by an accurate interface and the weld parameter for each paralleled power supply is provided by the general information network. This network can also operate a DC electrode which does not require the interconnected switching points used in the present invention.
In accordance with the present invention, there is provided an electric arc welding system for creating an AC welding arc between the electrode and workpiece. As will be explained, the system can drive one electrode by a single inverter. As a feature of the system, two or more power supplies can drive a single electrode. Thus, the system comprises a first controller for a first power supply to cause the first power supply to create an AC current between the electrode and workpiece by generating a switch signal with polarity reversing switching points in general timed relationship with respect to a given system synchronizing signal received by the first controller. This first controller is operated at first welding parameters in response to a set of first power supply specific parameter signals directed to the first controller. There is provided at least one slave controller for operating the slave power supply to create an AC current between the same electrode and workpiece by reversing polarity of the AC current at switching points. The slave controller operates at second weld parameters in response to the second set of power supply specific parameter signals to the slave controller. An information network connected to the first controller and the second or slave controller contains digital first and second power supply specific parameter signals for the two controllers and the system specific synchronizing signal. Thus, the controllers receive the parameter signals and the synchronizing signal from the information network, which may be an ethernet network with or without an internet link, or merely a local area network. The invention involves a digital interface connecting the first controller and the slave controller to control the switching points of the second or slave power supply by the switch signal from the first or master controller. In practice, the first controller starts a current reversal at a switch point. This event is transmitted at high accuracy to the slave controller to start its current reversal process. When each controller senses an arc current less than a given number, a xe2x80x9cready signalxe2x80x9d is created. After a xe2x80x9creadyxe2x80x9d signal from all paralleled power supplies, all power supplies reverse polarity. This occurs upon receipt of a strobe or look command each 25 xcexcs. Thus, the switching is in unison and has a delay of less than 25 xcexcs. Consequently, both of the controllers have interconnected data controlling the switching points of the AC current to the single electrode. The same controllers receive parameter information and a synchronizing signal from an information network which in practice comprises a combination of internet and ethernet or a local area ethernet network. In accordance with the invention, the timing accuracy of the digital interface is less than about 10 xcexcs and, preferably, in the general range of 1-5 xcexcs. Thus, the switching points for the two controllers driving a single electrode are commanded within less than 5 xcexcs. Then, switching actually occurs within 25 xcexcs. At the same time, relatively less time sensitive information is received from the information network also connected to the two controllers driving the AC current to a single electrode in a tandem welding operation. The 25 xcexcs maximum delay can be changed, but is less than the switch command accuracy.
In accordance with another aspect of the present invention there is provided an electrical arc welding system for creating an AC welding arc between an electrode and workpiece. The system comprises a first power supply to create a first AC current with first weld parameters between the electrode and workpiece by generating a first switch controlling signal reversing polarity of the first current at a specific switch time. A second power supply is provided to create a second AC current with second weld parameters between the same electrode and workpiece by a second switch controlling signal reversing polarity of the second current at a switch time coordinated with the specific switch time of the first power supply. The invention involves a timing interface between the first and second power supplies to create the second switch reversing signal by the first switch reversing signal where the switch signals are 10 xcexcs and preferably less than 5 xcexcs of the specific switch time. Consequently, the paralleled individually switched power supplies are coordinated by accurately matching the switch reversing times. The master controller has a switch command signal synchronized with a phase signal. The command signal is transmitted rapidly by the digital interface to the controller of the paralleled power supply. The second power supply then processes its switch point. In one embodiment, these switch points cause the reversal of polarity. Preferably, these switch points merely cause the inverters to be xe2x80x9ckilledxe2x80x9d so they decrease the currents by a time constant curve. When both currents are reduced below a given amount, the paralleled power supplies switch.
In the invention, the interconnected controllers have a polarity logic indicating the polarity of the two output currents. This merely assures that the two power supplies are switched with matching polarities. In this manner, the controller of the first power supply tells the controller of the second power supply which polarity is being reversed. The polarity logic is not a part of the invention although it is used in implementing the invention. The accuracy of the switching commands is the critical aspect of the digital high speed interconnecting interface between controllers that are otherwise controlled by an information network, such as an ethernet network with or without an internet link.
In accordance with still a further aspect of the invention, an electric arc welding system is provided for creating a first AC welding arc between a first weld electrode and a workpiece and a second AC welding arc between a second weld electrode and the same workpiece as the first and second electrodes move along the workpiece. This is the definition of a tandem mounted welding operation. The invention also comprises a system including a first cell with at least two power supplies connected to the first arc and operated at a first synchronized time determined by a first synchronized signal with first weld parameters and a high accuracy interconnection interface between the power supplies of the first cell to correlate polarity switching of the power supplies in the first cell. There is also provided a second cell with at least two power supplies connected to the second arc and operated at a second synchronized time determined by a second synchronized signal offset from the first synchronized signal with second weld parameters and a high accuracy interconnection interface between the power supplies of the second cell to correlate polarity switching of the power supplies of the second cell. A low accuracy information network, such as an internet link connected to a local area network, is connected to the first and second cells and contains digital signals including the first and second weld parameters and digitized first and second synchronizing signals. In this manner, the paralleled power supplies of each cell are interconnected by a high accuracy interface whereas the several controllers are operated with signals in the information network that are not time sensitive.
The primary object of the present invention is the provision of an arc welding cell or system including two parallel power supplies for driving a single electrode, wherein the power supplies are interconnected for coordinating the switching points or commands, but are independently controlled by an external network providing non-time sensitive parameters.
Another object of the present invention is the provision of the arc welding cell or system, as defined above, which cell or system can be used for driving two or more tandem mounted electrodes with offset phase relationship to prevent or reduce electrode interference.
Still a further object of the present invention is the provision of an arc welding cell or system, as defined above, which cell or system utilizes a series of smaller power supplies that are combinable to create a high capacity welding power supply for a single electrode. In this manner, several small power supplies can be combined to produce larger power supplies as greater current demands are experienced.
Still another object of the present invention is the provision of an arc welding cell or system, as defined above, which cell or system allows control of one power supply in the cell by parameters different from any other power supply. In this manner, one power supply can be maintained at a higher level while the other power supplies have a greater range for responding to larger current demands.
Another object of the present invention is the provision of an electric arc system including paralleled power supplies driving a single electrode with AC current, where there is matching of the polarity reversing switches to provide current added together at the electrode.
These and other objects and advantages will become apparent from the following description taken together with the accompanying drawings.